Power Supply Design - Reference Book

[TomCarlos]

MEF Members:

Long ago, I found a YouTube video that addressed Guitar Amp Power Supply Design. The interesting part was the fast and quick calculations one would use to determine requirements for the Preamp, Reverb, Tremolo, and Phase inverter circuits.

I struggle when looking at a schematic and trying to figure out what happens (in the voltage divider) after B+ and B++ (Plate and Screen taps points of the PS).

Using the Fender Princeton Reverb as an example, I cannot figure out what is the effective load at point "D" on the Power Supply. At that point in the circuit, we see 240 vdc. There is a 7025, two 12AX7, and one 12AT7. I see each of those circuits as a load (so to speak). Each tube (or half of the tube) has it's own current. So you add them all up (the current) and multiply times the effective resistance and you have a net resistor after point D? Is that how it works?

I am trying to find that video!! I have also been looking for reading material on this. Does anyone know of a reference (book, web site) that I can read up on the subject?

Thanks, Tom

[Enzo]

Um...

Look at schematic. it says 240v at D. Four branches, each with a 100k plate load to a 12AX7 plate. Each plate says 160v. Good, all same, so all same numbers. So in each case 80v dropped across 100k. I = E/R = 80/100k = 0.0008A = 0.8ma

Those add simply, so four times 0.8ma = 3.2ma. SO 3.2ma flowing through D. Oh wait, forgot the 40v dropped across 56k. I = E/R = 40/56k = 0.7ma. Add that is, so 3.9ma through D.

There is an implied and unused C node, so twice 80v across 18k, or 160v on 36k. We infer 160/36k = 4.4ma. That is a half ma discrepancy, but I'll write that off to circuit approximations, schematic roundings and other fudge factors. Call the whole thing 4ma.

A to B drops 20v across 1k, so 20ma. Since B is only the screens, we can subtract the 4ma from downstream leaving 16ma through the screens.

I suppose some would do it your way, but I have never needed to come up with a effective resistance. In my mind, typical 12AX7 circuits will average something like from half a mil to a little over a mil. SO I just use 1ma as a rule of thumb - close enough. Can always tweak it later.

DUncan Amps has a power supply calculator ; http://duncanamps.com/psud2/index.html

And I am sure Valve Wizard has things to say about it.

I don't know about videos.


Oh crap, I overlooked the reverb drive using B. Oh well, use the same math and stuff.

[TomCarlos]

An excellent explanation Enzo. Thank you!

I wondered about node C .... I guess this PS design is used on some other amp where node C was needed? In any event...

I did go through each piece of the preamp, trem, reverb, PI, etc and started adding up the current. I realized the three different tubes are pulling different current.

Perhaps the important things to know are this....

(1) What type of output tubes are being used, qty, etc and what is the desired plate voltage.
(2) Draw up the schematic for the preamp, trem, reverb, phase inverter and select the target voltage at D that will work for the circuit.
(3) Select a target voltage for D that will work for the lower voltage tubes. As shown on this schematic, the 12AT7 and one-half of a 12AX7 (tremolo) are at node B.
(4) Tweak as needed.

[TomCarlos]

There are some interesting online tools at Amp Books.com.

I tried the PS Design Simulation tool from Duncan Amps but the data and graph are not user friendly. I can't seem to get a graph that is shown in his example. Anyway, I will keep trying.

[Gingertube]

Hi Tom etc.,
The bypass caps for the preamp and phase splitter sections are not that critical whereas the caps for the output stage are more so.

For all my builds I have developed a system. I buy all Panasonic ED 22uF/450V caps and then just parallel them till the amp sounds fine. For 6V6 Push Pull I use 3 in parallel for 66uF.

For EL34 or 6L6 push pull I would start with 5 for 110uF and adjust to taste.

Cheers,
Ian